Patients with recurrent venous thromboembolic disease, disease at an early age, or a family history of this disease are at higher risk for recurrences of thrombosis; their family members are also potentially at higher risk than normal subjects. These patients are being studied for genetic abnormalities that may predispose them to thrombosis. These include abnormalities in the factor V gene (factor V Leiden), prothrombin gene abnormality 20210, and the mutation leading to labile 5, 10 methylenetetrahydrofolate reductase (which increases plasma levels of homocysteine, leading to thrombosis). DNA is isolated from peripheral blood leukocytes, and the DNA is analyzed using polymerase chain reaction and restriction enzyme techniques. Five patients have been studied thus far for these abnormalities.